Conventional cathode ray tubes suitable to color television include an envelope having a viewing area with an inner surface covered by phosphors to provide a correct color and image rendition when impinged by an electron beam emanating from an electron gun. Disposed intermediate the electron gun and the inner surface of the viewing area is an aperture mask having a plurality of closely spaced apertures. These apertures are of a dimension and configuration such that ideally the electron beam is prohibited from striking phosphors other than those which provide the correct and desired color and image rendition.
Normally, the above-mentioned aperture masks are made from a metallic material, such as steel, and the apertures are effected by way of a well-known photo-chemical machining process. Also, it has been a common practice to utilize a so-called phosphor-dot type screen wherein the aperture mask included a plurality of substantially circular holes whereby the phosphor dots were deposited onto the inner surface of the viewing area.
Accordingly, the dimensions of the holes and the resultant phosphor dots were critical in order to provide proper impingement of the phosphor dot by an electron beam. To insure the correctness of these critical dimensions, one technique provided a magnification apparatus whereby an inspector visually checked each aperture mask and passed or rejected the part. Obviously, such a technique is not only cumbersome and slow but also subject to operator error and judgement decisions.
Following, it was found that light transmission of the holes in the aperture mask provided information sufficient to accurately determine the hole size. Thus, the undesired relatively slow and relatively inaccurate magnification apparatus employing an operator and relying upon operator judgement was rendered obsolete and a faster more accurate light transmission technique evolved.
However, the phosphor-dot type of cathode ray tube is being rapidly replaced by the so-called "slotted" mask type of cathode ray tube structure. Therein, the apertures are in the form of slots, rather than holes, and the phosphors are in the form of stripes rather than dots. Thus, the new slotted arrangement requires measurements of both slot length and width if correct dimensional configurations are to be determined as compared with the relatively simple prior known holes wherein a single diametrical measurement was sufficient.
In an attempt to provide satisfactory measurements of the so-called "slots", a return was made to the so-called magnification process wherein a microscope and an operator were used to determine one dimension of the slot. Then, light transmission of the total slot was combined with this one microscopic measurement to determine the remaining dimension or width of the slots of the slotted aperture mask.
Although the above-mentioned technique has been and probably still is used with varying degrees of success, it has been found that the results leave much to be desired insofar as both accuracy and measurement efficiency are concerned. More specifically, it has been found that the technique again relies upon relatively inconsistent and inaccurate operator judgement. Moreover, undesired but relatively common variations and rounded ends of the slots render operator judgement and resultant accuracy difficult, if not impossible.
Thereafter, a comparator mask technique was developed as set forth in the above-mentioned co-pending application bearing U.S. Ser. No. 158,023 and incorporated herein by reference. Briefly, a comparator mask of alternate light transparent and opaque sectors is overlayed on a slotted apertured material such that the rounded ends of a number of slots are masked by the opaque sectors of the comparator mask. As a result, a given length of slot is exposed to the transparent sector of the mask and this given length of slot in conjunction with the amount of light passing therethrough, as provided and detected by oppositely disposed light source and light detector means provides signal information sufficient to determine the width of the slots under consideration.
Although the above-described comparator mask technique has provided greatly enhanced results, as compared with other known techniques, it was found that room for improvement still exists. Generally, it was found that an undesired fluctuation in light transmission readings was encountered. In other words, reproducibility of light transmission readings left something to be desired. For example, attempts to repeat a given light transmission reading at a previously designated area resulted in the above-described undesired reading fluctuation.
More specifically, it was discovered that utilizing a light beam which is substantially circular tends to provide a relatively large fluctuation in light transmission readings when an attempt is made to repeat the readings. Although no certainty exists, it is believed that this circular light beam configuration tends to provide a relatively large change in the amount of slot exposure for a small deviation in positioned location of the slotted material with respect to the light beam.
To illustrate, it may be assumed that a slotted apertured material includes a plurality of slots disposed within a substantially square beam of light. With each of the slots having a longitudinal axis extending normal to the one side of the squared beam of light, it can be seen that a slight deviation of the positional location of the light beam and slotted material with respect to one another would result in a relatively large change in light transmission through the slots. In other words, movement therebetween would cause a sudden shift in the number and amount of slots exposed to the relatively square beam of light and as a result, a relatively large and undesired fluctuation in light transmission readings.